Combustion turbines can be readily integrated with air separation systems to produce atmospheric gas products and electric power. The combustion turbine air compressor supplies compressed air for the turbine combustor and also may provide some or all of the compressed air feed to the air separation system. Typically the combustion turbine is integrated with a cryogenic air separation system to provide oxygen and/or nitrogen products, and the combustion turbine and air separation systems can be integrated with a gasification process in a gasification combined cycle power generation system.
The generation of electricity by advanced gasification combined cycle power generation systems offers the potential for reduced power cost and lower environmental impact than standard coal-fired power plants. In these advanced systems, coal or other carbonaceous material is gasified with oxygen and the produced gas is cleaned to yield a low-sulfur fuel gas. This fuel gas is utilized in a gas turbine generation system to produce electric power with reduced environmental emissions. Some or all of the nitrogen produced by the air separation system preferably is returned to the combustor to reduce the formation of nitrogen oxides and improve the efficiency of the integrated combustion turbine/cryogenic air separation system,
The growing interest in gasification combined cycle (GCC) technology in recent years has been stimulated by the higher efficiency and demonstrated reliability of advanced gas turbines, gasification processes, and air separation systems which are utilized in integrated gasification combined cycle (IGCC) systems. The proper integration of these three main components of an IGCC system is essential to achieve maximum operating efficiency and minimum power cost.
A general review of the current art in GCC and IGCC power generation systems is given by D. M. Todd in an article entitled "Clean Coal Technologies for Gas Turbines" presented at the GE Turbine State-of-the-Art Technology Seminar, July 1993, pp. 1-18. A. K. Artand et al present a review of the factors involved in the design of IGCC systems in a paper entitled "New Technology Trends for Improved IGCC System Performance" presented at the International Gas Turbine and Aeroengine Congress and Exposition, Houston, Tex., Jun. 5-8, 1995. A review of various integration techniques and the impact thereof on GCC economics is given in a paper by A. D. Rao et al entitled "Integration of Texaco TQ Gasification with Elevated Pressure ASU" presented at the 13.sup.th EPRI Conference on Gasification Power Plants, San Francisco, Calif., Oct. 19-21, 1994.
In a paper entitled "Improved IGCC Power Output and Economics Incorporating a Supplementary Gas Turbine" presented at the 13.sup.th EPRI Conference on Gasification Power Plants, San Francisco, Calif., Oct. 19-21, 1994, A. R. Smith et al review several modes of integration between the gas turbine and the air separation unit in an IGCC process. In one mode, the air separation unit feed air is provided by a separate compressor and a portion of the nitrogen product from the air separation unit is compressed and introduced into the gas turbine combustor. This nitrogen-integrated mode allows operation of the IGCC system at increased gas turbine power output and reduced NO.sub.x formation. In an alternative operating mode, nitrogen integration is combined with air integration in which a portion of the feed air to the air separation unit is supplied by extracted air from the gas turbine compressor. This alternative mode, defined as air and nitrogen integration, gives greater operating flexibility and allows for a higher degree of optimization during IGCC system operation at part load conditions.
Combustion-based power generation systems, including IGCC systems, are subject to periods of operation below system design capacity due to changes in ambient air temperature and/or the cyclic demand for electric power. During these periods, such systems operate below design efficiency. The equipment selection and process design of an IGCC system therefore must address steady-state operation at design capacity as well as operation at part load or turndown conditions. The air- and nitrogen-integrated IGCC system described above is a preferred option because of the potential for operating such a system at maximum overall efficiency, particularly when the system operates at part load or turndown conditions for significant periods.
Cryogenic air separation processes can be designed specifically for integration with combustion turbines or gas turbines, and air separation processes having both air and nitrogen integration with combustion turbines are of particular utility. Most air separation processes in this service utilize the well-known double column distillation system for efficient recovery of oxygen and nitrogen products.
U.S. Pat. No. 3,731,495 discloses an air- and nitrogen-integrated combustion turbine system in which a portion of the air from the gas turbine compressor is further compressed, treated to remove gaseous impurities, cooled, and introduced in total into the high pressure column of a double column distillation system.
U.S. Pat. No. 4,019,314 describes an air- and nitrogen-integrated steam generation/combustion turbine system in which a portion of the air from the gas turbine compressor is further compressed and introduced into an air separation unit. Oxygen product is used in a coal gasification system which generates fuel for the combustion turbine.
An air- and nitrogen-integrated combustion turbine system is disclosed by U.S. Pat. No. 4,224,045 in which a portion of the air from the combustion turbine compressor is cooled and purified in a reversing heat exchanger. The cooled purified air is divided into a first and a second portion, the first portion is introduced into the high pressure column of a double column distillation system, and the second portion is warmed, work-expanded, and introduced into the low pressure column. Optionally, the portion of air from the combustion turbine compressor is compressed further before cooling and purification. Optionally, the portion of air from the combustion turbine compressor is (1) work expanded before cooling and purification or (2) work expanded after cooling and purification but before dividing the air for introduction into the low and high pressure columns.
U.S. Pat. No. 4,557,735 describes an air- and nitrogen-integrated combustion turbine system in which a portion of the air from the combustion turbine compressor is purified by adsorption and cooled to provide the sole feed to a double column distillation system via the high pressure column. A nitrogen product from the high pressure column is expanded to provide refrigeration, utilized to regenerate the feed adsorption system, compressed, and introduced into the combustor of the combustion turbine system.
An air- and nitrogen-integrated combustion turbine system further integrated with a coal gasification system which provides fuel for the combustion turbine is described in U.S. Pat. No. 4,697,415. Extracted air from the combustion turbine compressor is introduced directly into the air separation system without either expansion or further compression.
U.S. Pat. No. 5,386,686 describes an air- and nitrogen-integrated combustion turbine system in which the oxygen content of the combined feed air and return nitrogen to the combustor is controlled by controlling the flow rate of the extracted air or the return nitrogen.
Air- and nitrogen-integrated combustion turbine systems are described in U.S. Pat. No. 5,406,786 in which the heat generated in feed air and oxygen product compression is used to humidify compressed air to the combustor of the combustion turbine system. The portion of air extracted from the combustion turbine compressor optionally is work-expanded before introduction into the air separation system.
Nitrogen-integrated combustion turbine systems are described in U.S. Pat. Nos. 5,081,845, 5,410,869, and 5,459,994, and in UK Patent Application No. GB 2 067 668 A.
The storage of one or more cryogenic liquids during periods of low product demand in low temperature air separation systems is described in U.S. Pat. Nos. 5,082,482, 5,084,081, and 5,224,336. During periods of high product demand the stored liquid is withdrawn as additional product or utilized within the air separation system as intercolumn feed or reflux.
High pressure oxygen and/or nitrogen products can be obtained by pumping liquid(s) withdrawn from a cryogenic air separation system and vaporizing the pressurized pumped liquids by heat transfer with cooling feed air streams as described by representative U.S. Pat. Nos. 5,098,457, 5,148,680, 5,303,556, and 5,355,682.
Combustion turbine systems currently in operation typically supply combustion air at pressures up to 200 psia. New combustion turbine systems recently have been introduced which operate at higher pressures in which the combustion air is provided in the range of 240 to 440 psia. These new high pressure turbine systems operate at higher efficiencies than the lower pressure systems currently in operation, and offer the potential for improved integration with cryogenic air separation systems. Improved methods for the integration of such high pressure combustion turbines with cryogenic air separation systems are described in the invention disclosed below and defined by the claims which follow.